Cowan, a Democrat, will hold the seat until the special election to fill Secretary of State John Kerry's Senate seat (primary April 30, general June 25).

Cowan's swearing-in makes him the eighth African-American senator in U.S. history. There are now two serving African-American senators, one Democrat and one Republican (South Carolina Sen. Tim Scott).

This is the first time in U.S. history that two African Americans have served in the Senate at the same time.

Scott, in fact, was the first senator to greet him on the Senate floor, as Cowan's hand came off the Bible. The two spent several minutes talking and laughing afterward.

There were smiles all around as Cowan was sworn in, first on the Senate floor -- the official moment -- and then in the "mock"ceremony before cameras in the Old Senate Chamber. Both were conducted by Biden.

Kevin Wolf / AP

Sen. William "Mo" Cowan, D-Mass., right, shakes hands with Secretary of State John Kerry in the Old Senate Chamber on Capitol Hill in Washington, Thursday, Feb. 7, 2013, following a re-enactment of his swearing-in as a Senator.

Cowan was escorted down to the well of the chamber by the man he will temporarily replace, Secretary of State Kerry, and his Massachusetts counterpart, Elizabeth Warren, now the senior senator from Massachusetts.

Cowan's former boss and the man who appointed him, Gov. Deval Patrick, was also on hand in the chamber. Prior to the ceremony, Kerry showed him around, pointing out his old desk and the contents of its drawer.

As Kerry moved on to attend to Cowan, Patrick had a seat at the desk.

Uh, not so fast.

The Sergeant-At-Arms approached and, according to Patrick later, told him he could stand on the chamber floor, but as a non-senator he was not permitted to sit there.

Recall that in his farewell last week, Kerry wept as he spoke of that same desk and how the Kennedy brothers, John F. Kennedy and Edward, had used it as senators. Patrick moved.

Then it was on to the mock ceremony, where Cowan and his family, including sons Miles, 8, and Grant, 4, waited along with Kerry and Warren for Biden. Miles, who informed Warren that he was deep into 'Diary of a Wimpy Kid,' was asked if he had anything to say.

"Fantastic!" he exclaimed to laughter.

A few minutes later, the veep arrived and again performed his duties, behaving himself this time and getting through the mock ceremony without shocking anyone.

"Its a great honor to be here," Biden told them. "I miss the place."

On his way out. Biden poked his head into Mitch McConnell's suite, grabbing some chocolate from the desk of a receptionist, then went around another corner and ended up in an embrace with a former staffer before leaving the Capitol.